Environmental aspects and the increasing price of the fossil fuels has increased the interest in renewable fuels in power production. One alternative, as a largely CO2 neutral fuel, is biomass. Biomass may be e.g. harvested as residues from forest industry. In addition, the biomass may comprise at least one of agricultural waste, peat, stubs, stumps, branches, and waste wood such as bark, wooden construction debris, and wood product residuals. Biomass may be used in a thermal process. E.g. biomass may be burned to produce energy. Alternatively, biomass may be gasified to produces synthesis gases, which may be further processed to biofuel. Still further, biomass may be treated in a pyrolysis process to produce pyrolysis gas, which may be condensed to pyrolysis oil. Still further, biomass may be treated in a torrefaction process to produce biocoke, which may be utilized elsewhere in combustion and/or gasification processes. These thermal processes produce a mixture of various gases, e.g. flue gas, synthesis gas, pyrolysis gas, or torrefaction gas. The content of a specific gaseous compound in the gases is often of interest because of various reasons. E.g. flue gases, as the results of combustion of biomass, cause corrosion and slagging problems in combustion boilers. One cause of the problems is alkali hydroxide vapors, which are formed during combustion. In order to quantify the corrosion problem, the hydroxide content of the flue gases needs to be measured. In addition to hydroxides, metal chlorides such as PbCl2, metal carbonates such as K2CO3, and metal sulfates such as K2SO4 may cause similar problems.
In general, optical spectroscopy may be utilized to measure the content of harmful gases from the gases. Methods include absorption spectroscopy, wherein the attenuation of light is measured. As different gaseous compounds absorb light differently, the attenuation spectrum may be used to deduce the content of different gaseous compounds. The sensitivity of the method is relatively low, being in the ppm (parts per million, 10−6) range. This is partly due to low absorption of the gases and partly due to fluctuations in the flue gas.
Optical methods include also those based on Excimer Laser Induced Fluorescence. In these methods, the alkali chloride molecules are dissociated, and the released alkali atom is excited using an excimer laser. As the alkali atom relaxes from the excited state, a photon is emitted. The wavelength of the photon corresponds to the alkali species and the alkali content is measured from the emission spectrum. In principle, the sensitivity of the method is in the ppb (parts per billion, 10−9) range. However, in a combustion environment, scattering of light e.g. from soot particles, dilutes the sensitivity of the methods.